I have a Trane central heating a/c unit (about 20 yrs. old). With the fan switch on AUTO the blower works normally in the a/c mode. In the heater mode, the burners fire but the fan does not come on. The fan operates with the heater on if I manually move the switch to the ON position. So I can assume that the motor/connections are okay.

My suspicion is that the heat activated relay switch has gone bad.

Question: If it is the replacement of a relay and/or heat probe, is this a DIY job or do I need to call a technician? Given a decent manual, I think I can do it (if that's all it is). What would be a good source for obtaining service manuals for specific models?

Thanks,
Tobor

 

After a while it overheats and the burner shuts down?



Might not even be a relay. See How to diagnose your blower problem.

It could be the fan/limit switch as seen in the first picture.
You can take the cover off, watch it work, probe the voltages.
Caution: fan leads are 120V (thermostat wires for limit are usually 24V).



This is a question only you can answer.



Manufacturer's website.

 

It could be two things..

1- Heating speed winding on the motor has gone out.
2- Fan limit/control is not closing. (Silver box with a big dial w/ 3 tabs settings)


If the fan runs with the t-stat switch fan to "ON", you got power.

You can leave it on to get heat.

Do you have a meter to take a reading to see if the fan motor is getting power on the heating speed tap?

 

Thanks for the replies, guys.

Bolide: I've never left it on (without the fan running) to see if it will shut down.

No, I don't own a meter. (Getting one would probaby be a good investment, though).

Jay: Getting into motor speed taps is, I think, a bit out of my league. I'll most likely wind up calling a technician. Currently, I do use the workaround of firing up the heater and manually switching the fan to ON.

I get heat, but since the fan is running at a higher speed, it tends to cool it a bit.

 

Yeah, if you think it's too much, best to call in service.. Hate to see you do more damage than good.

 

I wonder how you know that the fan wouldn't turn on once it gets hot enough?


A voltage probe is fast. Meters are under $10. The cheapest is all you need for the kind of diagnostics you are needing to do.

That won't hurt anything. I concur wath Jay about the two most likely causes.
(Except if yours is one of those time-delay controls rather than temperature-activated, then #2 is ruled out.)

 

Okay, let's say I get a meter and test it and it is sending voltage to the fan. I suppose I can assume that it's the heating speed winding. Right?

If it's not, then would the limit switch need adjusting or would it likely need replacing?

Bolide, I will try letting it run to see if it either shuts down or the fan starts.

 

If you do get a meter, you need to take a reading on heating tap on theblower termal to see if power is coming out.. If power is coming out, and fan is not running, then that winding has gone out.

If you don't have power coming, then have to trace back to the fan limit/control.

 

Okay, last night I did let it run and guess what? after about 3 min. the fan came on! (at the correct heating speed). So it's definitely not the motor winding, thank God!($$$).

The reason I felt there was a problem in the first place was that the fan used to come on almost immediately after the burners fired. I'd feel more comfortable if it still did that since an abrupt change in behavior indicates to me that something is not operating the same as before.

Should I fiddle with the limit switch settings to try to make it come on as it did before? Does the delay indicate a problem with or a deterioration of the limit switch? Should it be replaced?

Thanks for all your help.

 

I think that was the first question I asked.



> The reason I felt there was a problem in the first place was that the
> fan used to come on almost immediately after the burners fired.

What changed?


> I'd feel more comfortable if it still did that since an abrupt change
> in behavior indicates to me that something is not operating the
> same as before.

Please tell us the rest of the story.
Most of us do not babysit our furnaces and wouldn't notice if the fan starting turning on later.

So I'm thinking that you were bothering to notice.
What got your attention?



No, given the age, it sounds normal.



> Does the delay indicate a problem with or a deterioration of the limit switch?

It sounds normal to me.
If you have a computer control board with jumpers, the 30-second-delay jumper might be loose so it has changed to three-minute-delay.

Do you have a control board?
What model?


If it is temperature driven, it is fine.
There is nothing wrong with an old furnace heating up for a few minutes before the fan starts.
You feel less cold air that way.

 

Thanks for your patience Bolide. I really appreciate your knowledge and experience.

Actually, it was my wife who informed me that "Something is wrong with the furnace. I turned it on and it didn't come on". What she was really expressing was that the blower didn't come on quickly like it usually did.

Not knowing a darned thing about the inner workings of furnaces, -- [that was before. I have since educated myself a bit] -- I took this to be a sign of a problem since I didn't know any better. It was, all of a sudden, acting like it hadn't in the eight years I've lived in this house. After all, there's a big difference between 2 - 3 min. and "immediately". It never occurred to me to leave it on (as I thought it might be dangerous to do so).

I should note that I live in southern California in a pretty well insulated house. As such our central heater doesn't see that much action throughout the year. We usually get by with space heaters in the bedrooms.

I'm happy to report that since you encouraged me to leave it on a see what happens, we've been using it and it has been functioning normally (except the longer delay for the blower to kick in).

Thanks again for your help. Now I'm going to stop taking it for granted and take the vacuum to the blower motor and start using decent filters (instead of those el-cheapo blue mesh jobs).

 

Air quality is up to you. I say it matters more how often you change the filter than how expensive it is, at least as far as protecting the furnace goes.
A 50¢ changed monthly beats a $3.50 filter changed one a year.


Did you check into whether your control board has DIP switches or jumpers to set the blower delay?

 

I don't think a 20 years old furnace is going to have this.. Just the basic fan limit/control

 

You're right Jay. No control board. Just a junction box.

My limit switch can be seen here.

 

You can set the cut-in temperature to down to 130°F to get it to come on sooner.

 

I'll give it a shot. (Middle tab, right)?